Fertilizer is generated naturally by Followers in ways you surely can guess. Without an Outhouse, they will go to the edges of the area, away from their homes, and defecate in the wilderness, which can then be collected manually.
Outhouse II is even better at collecting crap, but remember to empty both manually and place the stuff in a Fertiliser Silo, so that farming followers will use it automatically.
Compost Bin and Natural Burial will convert Grass and dead Followers into fertilizer respectively.
Some of the more basic food types have a chance to make the follower poop instantly, which can be an useful side effect to get more fertilizer early on. However, one needs to be ready to react quickly, before the poop upsets other followers.
While the Janitor Station is useful to avoid health issues, no Fertilizer is gained when a follower cleans up poop. Followers will ignore other kind of Fertilizer, which allows the Lamb to collect them without risk of losing access to it.
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Wooden Planks
Stone Blocks
Spider Silk
Gold Bars
Coins
Crystal Shards
